300W Resistive load?
in [Design]
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From: JosephKK on 20 Jul 2010 08:21 On Sun, 18 Jul 2010 15:40:15 GMT, nico(a)puntnl.niks (Nico Coesel) wrote: >"Tim Williams" <tmoranwms(a)charter.net> wrote: > >>"Nico Coesel" <nico(a)puntnl.niks> wrote in message = >>news:4c42bd4b.224380781(a)news.planet.nl... >>>>What is something cheap and simple that can be used to dissapate 300W >>>>on a 200Vdc bus. I'm trying to think of something I could maybe pick >> ^^^^^^ >>>>up at a local homedepot or similiar. >>>> >>>>I need a 300W load to test out my PFC. >>>=20 >>> I've constructed an adjustable dummy load by using 4 TO3 transistors >>> from an old switching power supply. A heatsink and a couple of fans >>> take care of the heat. The whole thing isn't much larger than a shoe >>> box. >> >>Doesn't work in this case though, you'll be in the second breakdown = >>region. Unless it was slightly less old and used TO-3 MOSFETs, which = >>Idunno, might be worth something on eBay. :^) > >If you use transistors that can handle 200V (or way more) like I did >there is no problem. Plus it allows for all kinds of interesting test scenarios.
From: Jim Thompson on 20 Jul 2010 10:34 On Tue, 20 Jul 2010 05:18:16 -0700, "JosephKK"<quiettechblue(a)yahoo.com> wrote: >On Sun, 18 Jul 2010 08:28:44 -0700, Jim Thompson ><To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: > >>On Sun, 18 Jul 2010 08:05:57 -0400, Hammy <spam(a)spam.com> wrote: >> >>>On Sun, 18 Jul 2010 06:25:40 -0500, "Tim Williams" >>><tmoranwms(a)charter.net> wrote: >>> >>>>"Nico Coesel" <nico(a)puntnl.niks> wrote in message news:4c42bd4b.224380781(a)news.planet.nl... >>>>>>What is something cheap and simple that can be used to dissapate 300W >>>>>>on a 200Vdc bus. I'm trying to think of something I could maybe pick >>>> ^^^^^^ >>>>>>up at a local homedepot or similiar. >>>>>> >>>>>>I need a 300W load to test out my PFC. >>>>> >>>>> I've constructed an adjustable dummy load by using 4 TO3 transistors >>>>> from an old switching power supply. A heatsink and a couple of fans >>>>> take care of the heat. The whole thing isn't much larger than a shoe >>>>> box. >>>> >>>>Doesn't work in this case though, you'll be in the second breakdown region. Unless it was slightly less old and used TO-3 MOSFETs, which Idunno, might be worth something on eBay. :^) >>>> >>>>Hammy might have a few FETs left over from the boost though, they would handle the voltage just fine, as long as there's enough for the power. >>>> >>>>Tim >>>FETS I have a lot of. I also have some nice(2) brick heatsinks I >>>scavenged from some old microwave oven and car stero ampslifier. >>> >>>Thta was my original thought but I was thinking it would take to many >>>to dissapate the power 275W. >>> >>>The biggest I have in quantity is the FQA24N50 TO-3PN >>> >>>http://www.fairchildsemi.com/ds/FQ/FQA24N50.pdf >>> >>>I have about 30 of those. >>> >>> Actually those are in a pretty beefy package maybe 5 on a brick with >>>a fan might work. >>> >>> >>>I also have about 40 of these FQP27N25 TO-220 >>> >>>http://www.fairchildsemi.com/ds/FQ/FQP27N25.pdf >>> >>>Both of those Newark had on for dirt cheap and I was buying ten of >>>each everytime I did an order. Figured they might come in handy at >>>some time.;-) >>> >>>I have 1 or 2 IRF350 in to-3 no suitable heatsink for that power >>>level. >> >>Finned heat sinks, arranged as a square tunnel, fins inward, file, >>mill, whatever so they fit together nicely, and the "square" is >>muffin-fan size... which fits on one end. >> >> ...Jim Thompson >Yup. Also works fine with pairs and any air blocker to make a tunnel. >There will be a thermal gradient along the tunnel, with FETs this >works in your favor though. I had one power BJT per FACE... 400W audio amplifier ;-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 |
From: Nico Coesel on 20 Jul 2010 17:40 Grant <omg(a)grrr.id.au> wrote: >On Sun, 18 Jul 2010 20:27:27 GMT, nico(a)puntnl.niks (Nico Coesel) wrote: > >>"Tim Williams" <tmoranwms(a)charter.net> wrote: >> >>>"Nico Coesel" <nico(a)puntnl.niks> wrote in message = >>>news:4c431fff.249648812(a)news.planet.nl... >>>> If you use transistors that can handle 200V (or way more) like I did >>>> there is no problem. >>> >>>I'm just wondering if they were MOSFET or bipolar. TO-3 MOSFETs are = >>>somewhat old and rare. MJ15025 for instance won't handle more than = >>>0.17A at 200V, according to the SOA, which is only 34W, out of a rated = >>>250W. >> >>The ones I used are bipolar. I got them from a 150A or 300A 5V PSU. I >>don't know the part number but IIRC it is some sort of special type >>for switching purposes anyway. Each transistor is bolted to the >>heatsink directly for optimum heat transfer (the heatsink is live!). >> >>My dummy load actually is an adjustable current sink. A potmeter, a >>darlington emittor follower and series resistor make the drive for the >>'end stage'. >> >>The maximum load I ever tested was 400W. > >But not up at 200V? Low voltage, high current is easy, transistor >SOA at high voltage not so easy? I still fail to see why it wouldn't work. The only failure mode I can think of is the case in which the emitter resistors are too small to divide the load properly at low currents / high voltages. -- Failure does not prove something is impossible, failure simply indicates you are not using the right tools... nico(a)nctdevpuntnl (punt=.) --------------------------------------------------------------
From: Grant on 20 Jul 2010 20:29 On Tue, 20 Jul 2010 21:40:44 GMT, nico(a)puntnl.niks (Nico Coesel) wrote: >Grant <omg(a)grrr.id.au> wrote: > >>On Sun, 18 Jul 2010 20:27:27 GMT, nico(a)puntnl.niks (Nico Coesel) wrote: >> >>>"Tim Williams" <tmoranwms(a)charter.net> wrote: >>> >>>>"Nico Coesel" <nico(a)puntnl.niks> wrote in message = >>>>news:4c431fff.249648812(a)news.planet.nl... >>>>> If you use transistors that can handle 200V (or way more) like I did >>>>> there is no problem. >>>> >>>>I'm just wondering if they were MOSFET or bipolar. TO-3 MOSFETs are = >>>>somewhat old and rare. MJ15025 for instance won't handle more than = >>>>0.17A at 200V, according to the SOA, which is only 34W, out of a rated = >>>>250W. >>> >>>The ones I used are bipolar. I got them from a 150A or 300A 5V PSU. I >>>don't know the part number but IIRC it is some sort of special type >>>for switching purposes anyway. Each transistor is bolted to the >>>heatsink directly for optimum heat transfer (the heatsink is live!). >>> >>>My dummy load actually is an adjustable current sink. A potmeter, a >>>darlington emittor follower and series resistor make the drive for the >>>'end stage'. >>> >>>The maximum load I ever tested was 400W. >> >>But not up at 200V? Low voltage, high current is easy, transistor >>SOA at high voltage not so easy? > >I still fail to see why it wouldn't work. The only failure mode I can >think of is the case in which the emitter resistors are too small to >divide the load properly at low currents / high voltages. Not saying it cannot work, but you'd need big expensive transistors to make an active load for 200V at hundreds of watts. I'm working on a power DAC at the moment, only going up to about 30V and 30A, non-trivial when you want the same circuit to hold to the output to nearest mV or mA ;) First version with transistors leaked and drifted too much, current version being designed & built with MOSFET switched resistor banks and analog power amp to make up the small stuff seems far more viable. But here I'm drifting OT as I'm making a precision load, not a simple test load, but that explains why my focus is more on the reliability of an active load. Recently I built a 1A current sink, while it didn't fail, it was cooking a power MOSFET on large heatsink, but without fan, at only 30W. So I pulled the plug at 20 hours operation. Then realised I could use a bridge circuit instead of accurate current sink. Side stepped a problem, finding a better solution :) I ordered $5 worth of precision resistors 10:1 ratio. Grant.
From: Tim Williams on 21 Jul 2010 00:24
"Nico Coesel" <nico(a)puntnl.niks> wrote in message news:4c4616a1.443859171(a)news.planet.nl... >>But not up at 200V? Low voltage, high current is easy, transistor >>SOA at high voltage not so easy? > > I still fail to see why it wouldn't work. The only failure mode I can > think of is the case in which the emitter resistors are too small to > divide the load properly at low currents / high voltages. http://en.wikipedia.org/wiki/Avalanche_transistor#Second_breakdown_avalanche_mode So either the transistors seems to avalanche, i.e. current goes way higher than expected, or voltage starts pulling down, or it burns out altogether. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms |